A magnetic field is used in Magnetic Resonance Imaging to align the nuclear spins of atoms as part of the procedure for producing images within the body of a patient. This magnetic field is referred to as the BO field or main magnetic field. During an MRI scan, Radio Frequency (RF) pulses generated by a transmitter or amplifier and an antenna cause perturbations to the local magnetic field and can be used to manipulate the orientation of the nuclear spins relative to the BO field. Spatial encoding of the magnetic spins may be accomplished by using so called gradient coils, which are used to superimpose a magnetic field gradient upon the BO magnetic field. RF signals emitted by the nuclear spins are detected by a receiver coil, and these RF signals are used to construct the MRI images.
In order to perform magnetic resonance imaging, a superconducting magnet is typically used to generate the BO field. The BO field needs to have a large enough magnitude and uniformity to perform magnetic resonance imaging. The location where the magnet is able to generate a BO field of sufficient strength and uniformity is referred to herein as the homogeneous field zone.
Large body coils that are permanently mounted in the magnet are sometimes used to generate the RF pulses. In other cases smaller coils or antennas can be placed on or even attached to a subject. These smaller coils typically have a spatially dependent imaging zone where they are particularly sensitive for receiving a magnetic resonance signal and/or for transmitting a signal or RF pulse. To use such a smaller coil it should be positioned such that the imaging zone is within the homogeneous field zone of the magnet. Currently devices such as light visors are used to determine the location of the coil or antenna.
United States patent application US 20140055127 A1 discloses the use of wireless readable labels to determine the location of an antenna.
International patent application WO 2005/010544 A1 discloses, for a surface coil, applying gradient pulses in one or more directions followed by non-selective RF pulses. The location of the surface coil is then determined by calculating the center of gravity of the Fourier transformed response signals detected by the surface coil after applying non-selective RF pulses in each direction. The US-patent application US 2007/225588 mentions to employ a separate localisation device, notably based on fiducial markers, light reflecting markers, touch sensitive markers.